The invention relates in general to the technology of performing measurements on communication frequencies in a portable terminal of a cellular radio system. Especially the invention relates to the timing of such measurements that are performed on other frequencies than the one currently used for active communication.
In the technology of cellular radio systems, an interfrequency handover means that the frequency on which an active communication connection exists between a base station and a mobile station is changed. A cell change may accompany the interfrequency handover, in which case the maneuver is an intercell-interfrequency handover, or the frequency change may take place within a single cell meaning that an intracell-interfrequency handover is performed. The present invention is equally applicable to all interfrequency handover types.
In order to find a suitable target frequency for an interfrequency handover the mobile station must evaluate the available target frequencies in terms of connection quality that it could achieve on them. This in turn necessitates that the mobile station must quickly tune its radio receiver (or one of its radio receivers, in case it comprises several of them) onto each target frequency to be evaluated for a certain period of time. In TDMA (Time Division Multiple Access) systems this is not a problem since the mobile terminal must anyway transmit and receive only during certain cyclically occurring time intervals, between which it has time to tune its receiver onto whatever other frequencies it wants. However, in other systems like CDMA (Code Division Multiple Access) where reception and transmission are essentially continuous it may be problematic to find suitable time intervals for the measurements.
It is known to define and employ a so-called slotted mode for transmission and reception in order to leave certain time intervals free for measurement purposes. Slotted mode means that both transmission and reception are performed only according to a certain predefined slot pattern. FIG. 1 illustrates a set of frame trains of which frame train 101 corresponds to uplink transmission in normal mode, frame train 102 corresponds to downlink transmission in normal mode, frame train 103 corresponds to uplink transmission in slotted mode and frame train 104 corresponds to downlink transmission in slotted mode. The relative lengths of the slotted frames and the silent periods between them are defined in the applicable system specifications.
In a single-receiver station slotted receiving is essential in order to reserve the receiver to the use of the ongoing connection for only a part of the time. Slotted transmitting is not that essential at first sight, but usually it is unavoidable since the transmitter must be powered down for those time periods when the receiver is measuring. Leakage power from the transmitter might easily interfere with an ongoing measurement in the receiver.
Slotted mode is not without problems from the system point of view. Higher transmission power must be used in slotted mode than in continuous mode, since the closed-loop power control between the base station and the mobile station is not functioning properly and since the same amount of information must be sent in a shorter time. However, CDMA systems are extremely sensitive to increasing transmission power, because all simultaneously ongoing transmissions cause interference to each other. Additionally it may happen that the highest allowable transmission power levels were in use already in the continuous mode, in which case is not possible any more to increase the power like the slotted mode would require.
It is possible to avoid the last-mentioned problem by dimensioning the cells of a cellular radio system so that even at the outskirts of each cell there is a power marginal available for employing slotted mode. This means naturally that the operator responsible for the network must erect more base stations, which is economically unattractive.
It is an object of the present invention to provide a method and an arrangement for performing preparatory measurements for interfrequency handover with only a tolerable increase to the overall interference level in a cellular radio system. It is an additional object of the invention to provide such a method and arrangement without having to build an overdense network of base stations.
The objects of the invention are achieved by establishing a set of rules that determine, when a mobile station should go over to slotted mode in order to prepare for an interfrequency handover.
The method according to the invention is characterized in that it comprises the steps of
providing a set of criteria that define to be observed during a continuous communication mode
observing, whether at least one of said criteria is fulfilled during the continuous communication mode and
as a response to the fulfilment of at least one of said criteria is fulfilled during the continuous communication mode, changing the operation of a mobile station into a combined slotted communication mode and measurement mode.
The invention also applies to an arrangement that is characterized in that it comprises
means for storing a set of criteria to be observed during a continuous communication mode
means for observing, whether at least one of said criteria is fulfilled during the continuous communication mode and
means for reacting to an eventual fulfilment of at least one of said criteria during the continuous communication mode by changing the operation of the mobile station into a combined slotted communication mode and measurement mode.
The advantages of employing slotted mode are obvious from the point of view of making preparatory measurements for interfrequency handover, but unnecessary use of slotted mode must be avoided. After carefully considering the potential situations which may cause an interfrequency handover to be necessary we may present a set of rules that are most advantageously used in a mobile station to trigger the activation of slotted mode.
A first criterion which may cause a mobile station to go over to slotted mode and preparatory measurements for interfrequency handover is the reaching of the maximal transmission power allocated to the mobile terminal especially in the downlink direction. Reaching the maximal transmission power in the downlink direction is easily detected at the mobile station by observing that the received power level does not change remarkably despite of several successive power control commands where the mobile terminal asks for more downlink power.
A second criterion which should trigger the activation of slotted mode and preparatory measurements for interfrequency handover is an unusually high measured level of wideband interference. It is possible to lay down a set of simple rules to define what is unusual in this sense. It is known that a mobile terminal that happens to be very close to the base station it is actively communicating with (the xe2x80x9cservingxe2x80x9d base station) will experience a relatively high level of wideband interference, but at longer distances the importance of wideband interference should be lower. If, however, e.g. a pathloss measurement shows that the distance to the serving base station is long but the mobile station still experiences a high level of wideband interference, the origin thereof is most probably another base station or other transmitter operating on almost the same frequency. In such a situation it is usually advantageous to perform an interfrequency handover.
A third criterion for the same purpose comes from some estimates which the mobile terminal is able to make during normal operation. It is possible to estimate the interference level on a neighboring channel by subtracting from a measured narrowchannel interference level the estimated unorthogonal cochannel interference, the received thermal noise and the noise generated in the receiver itself. Combining this knowledge to location information e.g. in the form of pathloss measurements it is again possible to detect the presence of a nearby interfering transmitter on a neighboring channel. The mobile terminal may also store and analyze the characteristics of estimated unorthogonal interference compared to detected cochannel interference and apply a set of deduction rules to determine, whether or not there is a powerful interference source on a neighboring channel.